An alpha particle is a nucleus of helium (He), emitted during radioactive decay. Materials used to produce integrated circuits and integrated circuit packages and materials/contaminants inadvertently included in integrated circuits and integrated circuit packages during a manufacturing process on a specific production line may contain radioactive substances that emit alpha particles via radioactive decay. Examples of such radioactive materials include, but are not limited to, uranium impurities, thorium impurities (and their daughter products), and lead isotope Pb-210. These substances are capable of emitting alpha particles at parts per million (PPM) and parts per billion (PPB) levels. The kinetic energy of an emitted alpha particle is in the 4-9 MeV (million electron-volts) range. An emitted alpha particle can travel a distance of several centimeters, e.g., 2-7 cm, through air and can travel a distance of tens of microns, e.g., 20-100 μm, in silicon (SI).
With the ongoing dimension shrinkage of modern integrated circuits, transistor dimensions and transistor threshold voltages have been greatly reduced. As a result, modern integrated circuits with reduced dimensions have become increasingly sensitive to alpha radiation. For example, the radiation energy of alpha particles imparted to an integrated circuit transistor can cause an internal data state upset, e.g., a switch from a HIGH state to a LOW state, or vice versa, by generation of electrons and holes. Amongst the most vulnerable structures are those of high density transistor memories such as, for example, double data rate dynamic random access memory (DDR DRAM), double data rate synchronous dynamic random access memory (DDR SDRAM), as well as various flip-flop (FF) structures.
A data bit upset caused by impact of an alpha particle emission on a transistor within an integrated circuit does not cause physical damage to the transistor or to the integrated circuit. Further, the data bit upset, or data bit error, is recoverable using commonly used data processing techniques, such as data interleaving as well as data refresh. For this reason, such data bit errors are called soft errors. A soft error rate (SER) is the rate at which an integrated circuit is predicted to encounter soft errors. A soft error rate for a device can be expressed as either number of failures-in-time (FIT), or mean-time-between-failures (MTBF).
The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent the work is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.